1. Field of the Invention
The present invention relates to a sensing system using positive feedback.
2. Discussion of Related Art
In an existing water quality sensing system employing an actuator and a sensor, the magnitude of an input signal generated by the actuator is kept constant, and the sensor detects a change of a medium caused by the actuator. In a sensing system according to related art, the sensing system is configured to use a single ended signal scheme and/or use negative feedback scheme for a stable configuration.
As an example, in the case of a turbidity sensor, when the actuator radiates predetermined light to a medium including a material whose concentration will be measured, the sensor senses light passed through the medium and converts the light into an electrical signal, thereby measuring the concentration of the material included in the medium.
An existing sensing system has a limit of detection (LOD). For example, when the sensing system is used as a water quality sensor, even if a very small amount of material to be detected is included, that is, the amount of included material is less than the LOD, it is determined that the material is not included because the LOD characteristic of the sensing system is poor.
The present applicant applied for a patent for a sensing system which has an improved LOD to solve the aforementioned problem of the related art using positive feedback. However, according to the patent application, a photo-detector device which operates in a reverse bias state is used, and a voltage equal to or higher than a reverse breakdown voltage should be applied. In other words, the photo-detector device operates while the voltage equal to or higher than the reverse breakdown voltage is applied, and thus the device may be destroyed by the reverse breakdown voltage, that is, the problem of reliability may arise.
In addition, according to the patent application, it is possible to measure a concentration of a medium, etc. by measuring an equivalent resistance resulting from a current provided by the photo-detector and a both-end voltage, but the equivalent resistance may have a negative resistance value of giga-ohms or higher. Therefore, an expensive high-precision measuring instrument capable of measuring a resistance value of giga-ohms or higher is required.